1. Field of the Invention
The present invention generally relates to transmission power control in a wireless communication system adopting CDMA (Code Division Multiple Access). Especially, the present invention relates to a transmission power control apparatus and a transmission power control method for controlling transmission power in a base station.
In addition, the present invention relates to a mobile station communicating with the base station having the transmission power control apparatus.
2. Description of the Related Art
In the following, conventional transmission power control in a wireless communication system adopting CDMA will be described. A wireless communication system shown in FIG. 1 is taken as an example here. In the wireless communication system, a wireless network control station 1 controls a plurality of base stations 2, each base station 2 controls one cell 3 and the base station communicates with mobile stations 4 in the cell by wireless. The wireless network control station 1 is normally connected to the base stations 2 by cable.
Next, conventional transmission power control method will be described by using FIG. 2. So-called closed loop transmission control performed between the base station and the mobile station will be described.
In the mobile station 4, a signal sent from the base station 2 is received by a receiver 11. Then, SIR (Signal/Interference Ratio) of the received signal is measured by a received SIR measuring part 12.
The received SIR is compared with a target SIR which is previously held by a comparing circuit 13 so that transmission power control information is generated on the basis of the result of comparison. The transmission power control information (normally included in TPC bits) is sent to the base station 1102 by a transmitter 14.
A receiver 15 of the base station 2 receives the transmission power control information, and sends the transmission power control information to a transmission power control circuit 16. The transmission power control circuit 16 changes transmission power of the transmitter 17 according to the transmission power control information.
In the base station 2, the receiver 15 receives a signal sent from the mobile station 4, and SIR of the received signal is measured by a received SIR measuring part 18.
The received SIR is compared with a target SIR which is previously held by a comparing circuit 19 so that transmission power control information is generated on the basis of the result of comparison. The transmission power control information (normally included in TPC bit) is sent to the mobile station 4 by the transmitter 17.
A receiver 11 of the mobile station 4 receives the transmission power control information, and sends the transmission power control information to a transmission power control circuit 20. The transmission power control circuit 20 changes transmission power of the transmitter 14 according to the transmission power control information.
As mentioned above, according to the conventional transmission power control method, the base station and the mobile station monitors each other, and perform adjustment of transmission power with reference to SIR. That is, when the received SIR is smaller than the target SIR, the base station or the mobile station instructs the other end station to increase transmission power. On the other hand, when the received SIR is larger than the target SIR, the base station or the mobile station instructs the other end station to decrease transmission power.
In addition, the conventional transmission power control apparatus sets an upper limit value and a lower limit value of transmission power which can be used by a call (or a user) when establishing down-link.
The reason for setting the upper limit value and the lower limit value is (1) to prevent a call (or a user) from occupying transmission power more that a predetermined value, (2) to limit interference power given to other call, and (3) to stabilize the mechanism of the transmission power control.
The setting of the upper limit value and the lower limit value will be described with reference to FIGS. 3A and 3B. FIG. 3A is a graph schematically showing an example of the upper limit value and the lower limit value set for a call. FIG. 3B is a graph showing an example of transition of transmission power with respect to time. As shown in FIG. 3A, the upper limit value of transmission power occupied by a call is set to be 30% of the maximum power which can be transmitted by the base station, and the lower limit value is set to be 10% of the maximum power which can be transmitted by the base station.
Effects of the upper limit value and the lower limit value exerted on the transmission power of the call will be described by using FIG. 3B. The transmission power changes according to before mentioned transmission power control with respect to time. However, when a power value exceeding the upper limit value (30%) should be set according to the transmission power control, the upper limit value functions so as to reduce the transmission power below the upper limit value as shown in the figure.
In addition to controlling the power value by using the upper limit value, the conventional transmission power control apparatus performs control of suppressing over-input to a transmission amplifier. That is, when many calls are temporarily connected, although transmission power of each call is within the upper limit value, there is a case in which total transmission power of the calls exceeds the maximum allowable input value of the transmission amplifier. For solving this problem, a part for reducing total transmission power is provided before the transmission amplifier to prevent over-input.
This reducing control will be described with reference to FIGS. 4A–4C. In this example, four calls (calls 1–4) are connected in which call 1 and call 2 are circuit switching type calls, and call 3 and call 4 are packet switching type calls, and the upper limit value of each call is 5. When transmission power of each call changes while being controlled by the upper limit value as shown in FIG. 4A, the total sum of the transmission power of the calls is as shown in FIG. 4B.
If the maximum allowable input value of the transmission amplifier is 14, the upper limit value of the transmission power shown in FIG. 4B becomes 14 so that the transmission power is to be reduced below 14 if it exceeds 14. FIG. 4C shows a graph after the transmission power is reduced. As shown in FIG. 4C, parts where transmission power exceeds 14 in FIG. 4B becomes flat at 14.
As mentioned above, according to the conventional transmission control apparatus prevents over-input to the transmission amplifier by transmission power control on the basis of SIR by reducing the transmission power before the transmission amplifier.
However, according to the conventional transmission power control, when the total transmission power reaches the maximum allowable input value as a result of changing transmission power of each call, the transmission power of each call in the multiplexed calls is lowered uniformly and evenly. Therefore, there is a problem in that requirement for each call can not be reflected so that communication quality of the whole system deteriorates.
In the following, the conventional technology will be described from another aspect.
In a wireless communication system adopting W-CDMA, distinction between channels is performed by code instead of frequency in which all users share the same frequency.
For example, when considering communication between the base station and a plurality of mobile stations, communication capacity in the above-mentioned system is determined by transmission power of each mobile station since all mobile stations share the same frequency.
If each mobile station performs transmission by using a predetermined fixed power, excessive transmission power may be used depending on a place where communication is performed. This is inefficient.
Thus, in a conventional system, high speed transmission power control (Transmitter Power Control: TPC) is performed in up-link and down-link between the base station and the mobile station to change transmission power of each mobile station every moment. More particularly, transmission power of each mobile station is controlled such that each call has a SIR by measuring SIR (Signal/Interference Ratio) of a communication channel.
As mentioned before, this control is normally performed in the following way. The mobile station measures SIR of a call used by the mobile station itself, calculates difference between the measured SIR and a predetermined SIR (a target SIR). Then, the mobile station sends, to the base station, a transmission power increase or decrease request (TPC command) for reducing the above-mentioned difference by using TPC bits inserted into data header of the up-link channel. After that, the base station increases or decreases the amplitude of transmission signal according to the command (TPC command).
According to the transmission power control, SIR of each call becomes constant, and transmission by excessive transmission power more than enough for satisfying the SIR is not performed so that communication quality and channel efficiency improve.
Even in TPC, as for a control channel such as CCCH (common control channel) which is commonly used by a plurality of users, the transmission power is kept constant.
However, according to the conventional transmission power control (TPC), when the transmission power of the base station reaches the maximum allowable value as a result of performing increase or decrease of amplitude of transmission signal of each call according to TPC, transmission power of each call is lowered evenly and uniformly. Therefore, there is a problem in that requirement for each call can not be reflected so that communication quality of the system deteriorates as a whole as mentioned above.
In the following, the problem will be described in detail with reference to FIGS. 5–7.
Regarding a down-link from the base station to the mobile station, transmission power of the base station has a limitation due to performance of the base station. As the number of connecting mobile stations increases, the transmission power increases consequentially. Therefore, the limitation of the transmission power determines the number of mobile stations which can be connected to the base station. In any way, there may be a case in which the transmission power reaches the maximum allowable value which can not be increased any more as a result of increasing or decreasing the transmission power of the base station according to TPC regardless of the number of the mobile stations.
Since TPC continues to function as before even in the above-mentioned situation, the transmission power may exceeds the maximum allowable value according to TPC so that the amplifier may be damaged. In order to solve this problem, an over-input reducing part, which is a limiter for example, is provided before a transmission amplifier such that transmission power larger than a allowable value is not applied to the transmission amplifier in order to protect the transmission amplifier from damaging. In the following, a conventional transmission power control apparatus will be described with reference to FIG. 5.
FIG. 5 is a block diagram showing only components relating to the present invention in a conventional transmission power control apparatus 600. In this case, the transmission power control apparatus is included in the base station. The transmission power control apparatus 30 includes a plurality of (for example N) baseband signal processing parts 31, transmission power control parts 32 each provided after the baseband signal processing part 31, a baseband signal multiplexing part 33 for multiplexing baseband transmission signals of each call, an over-input control part 34 which is a limiter for example, a power amplifier 35 and an antenna 36.
Each baseband signal processing part 31 performs baseband processing on user data to be transmitted for each call.
Each transmission power control part 32 increases or decreases amplitude for each transmission signal which is baseband-processed according to a TPC command from up-link.
The baseband signal multiplexing part 33 multiplexes transmission signals of each call.
The over-input reducing part 34 is a limiter for example, and, in order to protect the power amplifier 35 from damaging, the over-input reducing part 34 reduces the amplitude of the multiplexed transmission signal below a predetermined value such that a signal having power value larger than an input allowable power value of the power amplifier is not input into the power amplifier 35.
The power amplifier 35 amplifies a multiplexed transmission signal by a constant gain. The antenna radiates the transmission signal.
Next, the above-mentioned power reducing will be described with reference to FIGS. 6 and 7. FIG. 6 shows a schematic graph for a case when transmission power reducing is not performed for transmission power of the base station. FIG. 7 shows a schematic graph for a case when the transmission power reducing is performed by the conventional apparatus. In this example, signals are transmitted for calls 1–5, and the maximum allowable transmission power of the base station is 10 on the vertical axis of the graph. Then, it can be recognized that the transmission power exceeds the maximum allowable value at times 2, 3 and 6.
According to the conventional apparatus, since the total transmission power of the transmission signals of every call is reduced as a whole before the transmission amplifier, the transmission power of each call is reduced evenly, that is, in the same ratio.
That is, since the conventional apparatus reduces the total transmission power as a whole before the transmission amplifier, the transmission power of each transmission signal is reduced evenly, that is, in a same ratio. Therefore, quality deterioration and the like may be caused to every call. Although decreasing the transmission power evenly, for each call may be fair, type of circuit of each call is not considered by this method so that the system becomes inefficient as a whole and can not provide a kind communication service for users.
For example, as for voice communication which is performed by circuit switching for realizing real-time communication, it is practically difficult to compensate for quality deterioration by retransmission and the like, in addition, when communication stops due to quality deterioration, a serious problem occurs if the communication is for emergency (for example, communication from an ambulance carrying a sick person to a hospital.).
Accordingly, in a wireless communication system accommodating a plurality of types of circuits having various requirements, the conventional method in which the total transmission power is lowered as a whole without consideration of types of circuit is not efficient (especially when a call of the circuit switching type is included), and using the conventional method may lead to deterioration of communication quality as a whole system.